Longman Dictionary of Contemporary English
Wiktionary
n. A four-wheeled vehicle with a drivetrain that allows all four wheels to receive torque from the engine simultaneously.
WordNet
n. a motor vehicle with a four-wheel drive transmission system [syn: 4WD]
a transmission that provides power directly to all four wheels of a motor vehicle [syn: 4WD]
Wikipedia
Four-wheel drive, 4×4 ("four by four"), and 4WD, is a form of drivetrain capable of providing power to all wheel ends of a two-axled vehicle simultaneously. It may be full-time, or on-demand, and is typically linked via a transfer case which provides an additional output drive-shaft, along with additional gear ranges.
A four-wheeled vehicle with power supplied to both axles may sometimes be described as "all-wheel drive" (AWD). However, "four-wheel drive" typically refers to a set of specific components and functions, and/or intended offroad application, which generally complies with modern use of the terminology.
Usage examples of "four-wheel drive".
Dale tried to estimate the age of the pickups and whether they had four-wheel drive.
I crunched down into low gear and went into four-wheel drive as the track snaked its way up the cliffs, doubling back on itself so often that I had a zany idea I might drive into my own rear.